The present invention relates to a self-contained apparatus designed to sharpen general purpose right hand twist drills approximately one-eighth inch to approximately one-half inch in diameter to commercial tolerances. A large number of prior art twist drill grinding machines are known in the art. These machines grind the points on twist drills or the like to desired contours by various methods. The present apparatus relates to a type of machine which grinds a cylindrical profile on each side of the leading end of the twist drill to either side of the web. One prior art patent which discloses an apparatus for grinding a cylindrical profile in U.S. Pat. No. 1,674,224 to Rabut issued June 19, 1928. The Rabut patent disclosed a beltdriven grinding wheel, a chuck which is operable to support twist drills or the like with the leading ends of the twist drill in contact with a surface of the grinding wheel, and means to support the chuck relative to the grinding wheel. The supporting means include a frame which is pivotally secured to the base which also supports the grinding wheel, a carrier which is pivotally secured to the frame, a shaft journaled for rotation within the carrier, and an arm which extends away at an angle from the shaft, the arm in turn being interconnected with the chuck. A gauge is also mounted adjacent one end of the arm and means are provided for elevating and depressing the chuck and the gauge to accommodate twist drills of different sizes. The chuck is mounted in such a manner that it can be rotated or indexed about its center line 180 degrees between two working positions.
While the apparatus shown in the Rabut patent will satisfactorily grind the leading end of twist drills, it has numerous disadvantages. During the operation of the grinding apparatus shown in Rabut, it will be occasionally necessary to dress the grinding wheel to level off the surface which contacts the leading end of the twist drill. However, Rabut does not disclose any structure whereby a dressing tool could be mounted in his apparatus for dressing the grinding wheel, and thus, it would be necessary to provide additional means for dressing the grinding wheel. While a dressing tool could be mounted on the base of the Rabut machine in a manner taught by the U.S. Pat. No. 3,651,602 issued Mar. 28, 1972 to Hillier, such a device unnecessarily complicates the structure shown in Rabut and furthermore is somewhat difficult to operate so that the surface of the wheel which contacts the leading end of the twist drill lies in a surface parallel to the axis of rotation of the shaft which supports the chuck. Alternatively, a dressing tool could be carried by the chuck support in the manner taught by the U.S. Pat. No. 3,132,450 issued May 12, 1969 to Boddgert. While this design eliminates some of the disadvantages of Hillier, it unnecessarily complicates the design of the chuck support.
A further disadvantage of the apparatus shown in the Rabut patent is that it is difficult to provide for generally uniform wear of the grinding wheel. Thus, in order to present the leading end of the twist drill to different portions of the grinding surface of the grinding wheel, it is necessary to adjust either the frame which carriers the chuck or to longitudinally adjust the shaft about which the chuck is swung during the grinding of the twist drills.
Another disadvantage of the Rabut construction is that the shaft about which the chuck rotates must be shifted toward and away from the axis of the grinding wheel during the grinding of the leading end of a twist drill and must be also shifted as the diameter of the grinding wheel varies. In order to maintain the proper geometry on the leading end of the twist drill during the grinding operation, it is therefore necessary to very accurately machine the bearings within the pivoting frame and the pivotal carrier, and it is also necessary to very accurately position the pivoting carrier on the frame during the grinding operation.
An additional disadvantage of the Rabut construction is that it requires adjustment of the gauge and chuck for twist drills of different diameters. Also, the gauge of Rabut only initially positions the point of the twist drill and does not serve as a final gauge to properly locate the twist drill during the final grinding operation on the leading end. In this connection, it should be noted that it may sometimes be necessary to grind a large quantity away from the leading end of the twist drill, either because of severe wear on the leading end of the drill or because of a broken point. In this case, because the flutes are spirally disposed about the drill, the final position will not in any way correspond to the initially engaged position. While other gauges are well known in the prior art, as for example U.S. Pat. No. 2,614,370 issued Oct. 21, 1952 to Kapnick, devices of this type do not overcome the foregoing disadvantages.
A further disadvantage of the Rabut construction is that it is not a self-contained unit and therefore must be mounted in such a manner that it can be driven from a power take-off shaft within the shop through a belt or the like.